The Master Ball isn't a necessity, since any Pokemon can be caught without it. Use it when you have spent around three days (a couple hours a day) trying to catch a rare Pokemon (legendary only) with Ultra Balls at low HP and with a status problem. It is wise to do so, since you may find that a Pokemon may be tough to catch and you might not have your Master Ball.
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No, you do not need a master ball, if you get it to a low enough hp, you can catch it with an ultra ball. You can find mew in the end of the cave in cerulean.
No but you can if you want to but its stupid
black apricorns you need to exchange it with Kurt in azeala town in Pokemon soulsilver and heartgold
You don't really need a Master Ball to get a legendary. However, the only other way to get a Master Ball legitimately is by the Jubilife City lottery. If all of the numbers match, you get a Master Ball.
You'll need to trade for it from Pokemon HeartGold. It is not avilable in the wild in Pokemon SoulSilver.